Methods and apparatus providing thermal isolation of photonic devices

ABSTRACT

Described embodiments include photonic integrated circuits and systems with photonic devices, including thermal isolation regions for the photonic devices. Methods of fabricating such circuits and systems are also described.

CROSS-REFERENCES TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.13/524,446, filed Jun. 15, 2012, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

Described embodiments relate generally to the field of electronicphotonic devices, and more particularly to the thermal isolation ofelectronic-photonic devices.

BACKGROUND

Optical transmission may be used as a means for communication betweenseparate integrated circuit die (also referred to as inter-dieconnections), and between components on the same die (also referred toas intra-die connections). Photonic devices are a class of devices thatare capable of sourcing, controlling, and/or detecting opticaltransmission of signals.

The term “silicon photonics” relates to the study and application ofphotonic systems—that use silicon as an optical medium. Instead of, orin addition to, using silicon to facilitate the flow of electricity,silicon can be used to direct the flow of photons or light. Silicon istransparent to infrared light with wavelengths above about 1.1micrometers. Silicon also has a high refractive index of about 3.5. Thetight optical confinement provided by this high index allows formicroscopic optical waveguides, which may have cross-sectionaldimensions of only a few hundred nanometers, thus facilitatingintegration with current nano-scale semiconductor technologies that alsoemploy silicon, such as complementary metal oxide semiconductor (CMOS)technologies. Silicon photonic devices can thus be made using existingsemiconductor fabrication techniques. Further, because silicon isalready used as a substrate for many electronic integrated circuits, itis possible to create hybrid devices employing both optical andelectronic components integrated onto a single die.

In response to more demanding communication bandwidth, energyconsumption, and performance standards for electronic devices such assemiconductor devices, photonic devices are increasingly beingintegrated with optical/electrical circuits to form a type ofelectronic-photonic device called an electronic-photonic integratedcircuit. For example, in the semiconductor industry, photonic deviceshave various applications including communication within a die, betweenmultiple die of a computer board, and between computer boards.

In inter-die communications via optical interconnects, each die on thecircuit board can be interfaced with a photonic-electronictransmitter-receiver circuit, with two die operably connected via anoptical waveguide. Likewise, in intra-die communications, opticalwaveguides may be used to connect components within a die, such asbetween an integrated optical source and a photonic detector.

FIG. 1 illustrates a block diagram of one example of a conventionalphotonic multiplexing system 100. The system 100 includes multiplecarrier wave input devices 110 a, 110 b, 110 c, 110 d (generallyreferred to as an input device 110), each of which may be, for example,an optical source configured to generate an optical carrier wave at arespective transmission wavelength. For example, each input device 110may include a coherent light source, such as a laser (e.g., a hybridsilicon laser or a gallium arsenide laser), or other appropriate lightsource known in the art.

The optical carrier wave from each input device 110 a-d is transmittedto a respective resonant carrier wave modulator 120 a-d along arespective optical waveguide 115 a-d. Carrier wave modulators 120 a-dare configured to receive respective optical carrier waves havingdifferent wavelengths from the respective input devices 110 a-d; and tomodulate data on the optical carrier wave that it receives. For example,carrier wave modulators 120 a-d may be optical modulators configured toreceive an optical data signal and output a modulated optical datasignal, or electro-optical modulators configured to receive anelectrical data signal from an electrically conductive interconnect andoutput a modulated optical data signal. The modulated light from each ofthe carrier wave modulators 120 a-d is then combined and transmittedonto a single transmission channel (e.g., optical waveguide 140) usingan optical multiplexer 130. The multiplexed light is transmitted alongoptical waveguide 140 to an endpoint (not shown) that may include, e.g.,one or more photonic detectors for detecting optical transmissions,where the light is de-multiplexed and demodulated before being used byan endpoint device.

Wave guiding of an optical carrier wave through optical waveguides 115,140 occurs through internal reflection of electromagnetic waves of anoptical carrier wave at the interface between a higher refractive indexinner core and a lower refractive index outer cladding. For example, theinner core of an optical waveguide 115 may be formed of a silicon (Si)or silicon-containing material, and may have a refractive index ofapproximately 3.5. The cladding of an optical waveguide 115 may beformed of a material having a lower index of refraction, for example, aSiO₂ material with a refractive index of approximately 1.5.

Several components within a photonic system, and particularly componentsoperating at a resonance frequency, can be affected by variations intemperature. Variations in temperature can result in changes in thedevice dimensions (due to thermal expansion) and refractive indices ofmaterials. As one example, an optical laser providing one or morecarrier wavelengths can be tuned by changing its temperature. As anotherexample, changes in temperature can affect the operation of a resonantcarrier wave modulator 120. The resonant frequency of a particularmodulator 120 is controlled in part by the refractive indices of itsresonant structures, which may change according to temperature,resulting in turn in a deviation of the resonant frequency of themodulator 120. Accordingly, certain photonic devices require a stablethermal environment to perform optimally.

One technique for providing a stable thermal environment for photonicdevices includes active temperature control of one or more photonicdevices, such as through an electric heating device. FIG. 2A shows atop-down view of photonic devices including an input device 110 (e.g., alaser), an optical waveguide 115, and a resonant carrier wave modulator120, formed in a portion of a silicon die 230. A heating device 212provides active temperature control of carrier wave modulator 120.Heating device 212 may be, for example, a resistive or inductiveelement, such as a polysilicon, silicon, or copper element, that isconfigured to receive energy (e.g., electrical energy) and output heatto the surrounding photonic devices. As shown in FIG. 2A, heating device212 may be customized to modulator 120, by partially encirclingmodulator 120, or may have any other shape and be located near othertemperature-sensitive photonic devices, as well as carrier wavemodulator 120. In other embodiments, heating device 212 may beintegrated with carrier wave modulator 120.

FIG. 2B shows a cross-sectional view of photonic devices formed in aportion of a silicon on insulator (SOI) integrated circuit die 230. Die230 includes an input device 110 (e.g., a laser), an optical waveguide115, and a resonant carrier wave modulator 120. Die 230 includes asubstrate 232, which may be, for example, a bulk region of thermallyconductive silicon. The SOI structure also includes an insulator region233 (also referred to as a buried oxide or “BOX” region) composed of aninsulating material, such as SiO₂, which acts as a bottom cladding layerfor an optical waveguide 115. Die 230 may also include an interleveldielectric (ILD) region 236 having a lower layer composed of, forexample, SiO₂, formed above the device formation layer 235. The lowerlayer of ILD region 236 provides an upper cladding region for opticalwaveguide 115, and upper levels of the ILD region 236 are used forforming electrical connections in various locations of die 230, such asabove the device formation region 235. Typically, photonic devices,including input device 110 (e.g., a laser), optical waveguide 115, andresonant carrier wave modulator 120, as well as heating device 212, areformed in the device formation region 235 above the substrate 232.Device formation region 235 may include regions of silicon for formingphotonic devices, such as the inner core of optical waveguide 115 andmodulator 120, and regions of a cladding material, such as SiO₂, on thesides of optical waveguide 115 to serve as cladding surrounding asilicon waveguide core, and to serve as an insulating and mechanicallysupportive material for the devices formed in device formation region235. Other photonic devices, such as other optical waveguides, lasers,filters, or photonic detectors, may also be formed in the deviceformation region 235, and may be subjected to active temperature controlusing a heating device 212.

Referring to FIG. 2B, while heating device 212 provides activetemperature control of one or more photonic devices, such as modulator120, in device formation region 235, it emits heat q that dissipates inall directions, including into the substrate 232, which is formed ofthermally conductive silicon, and into the ILD region 236. This resultsin wasted heat flux and a less efficient heating device 212.Additionally, photonic devices in die 230 are typically thermallycoupled to the substrate 232, and any global temperature variation inthe die 230 may have an effect on the photonic devices.

Accordingly, it is desirable to thermally isolate heating devices andphotonic devices in a photonic system, in order to improve efficiencyand to provide more stable operation of the photonic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a conventional photonic system.

FIGS. 2A and 2B illustrate a top-down view and a cross-sectional view,respectively, of photonic devices in a die.

FIGS. 3A-3E illustrate a top-down view and cross-sectional views,respectively, of die including thermal isolation regions, in accordancewith embodiments described herein.

FIGS. 4A-4G illustrate processes for forming a die including a thermalisolation region, in accordance with embodiments described herein.

FIG. 5 illustrates a cross-sectional view of a die including a thermalisolation region, in accordance with embodiments described herein

FIGS. 6A-6E illustrate a top-down view, cross-sectional views, and athree-dimensional view, respectively, of die including thermal isolationregions, in accordance with embodiments described herein.

FIGS. 7A-7E illustrate processes for forming a die including a thermalisolation region, in accordance with embodiments described herein.

FIG. 8 illustrates a processor system, in accordance with embodimentsdescribed herein.

DETAILED DESCRIPTION

In the following detailed description, reference is made to variousembodiments. These embodiments are described with sufficient detail toenable those skilled in the art to practice them. It is to be understoodthat other embodiments may be employed, and that various structural,logical and electrical changes may be made. In addition, where variousprocesses are described, it should be understood that the steps of theprocesses may occur in an order other than how they are specificallydescribed, unless noted otherwise.

Embodiments described herein include photonics devices and integratedcircuits with a thermal isolation region for controlling dissipation ofheat in a photonic system. For example, a thermal isolation region maybe formed near one or more temperature-sensitive photonic structures.

FIGS. 3A and 3B illustrate one embodiment. They show a top-down view anda cross-sectional view, respectively, of photonic devices formed in aportion of a die 330. As shown in FIG. 3A, die 330 may include one ormore temperature sensitive photonic devices of a photonic system,including, for example, a resonant carrier wave modulator 320, such as aring resonator or disk resonator, or another type of resonant modulator.Carrier wave modulator 320 may be an electro-optical modulatorconfigured to receive an electrical data signal from an electricallyconnected interconnect, such as an electrically conductive signal line,and modulate a carrier wave in an adjacent optical waveguide 315according to the received data. Alternatively, carrier wave modulator320 may be an optical modulator configured to receive an optical datasignal, such as from another optical waveguide.

Die 330 also includes one or more heating devices 312 providing activetemperature control of one or more of the photonic devices, such as thecarrier wave modulator 320. Heating device 312 can be a resistiveelement, such as a polysilicon, silicon, or copper element, that isconfigured to receive electrical energy from an electrically connectedinterconnect, such as a via, and output heat. Although carrier wavemodulator 320 and heating device 312 are shown as separate elements FIG.3A, it should be understood that heating device 312 may also beintegrated with carrier wave modulator 320.

Die 330 also includes a carrier wave input device 310, such as a laserconfigured to output an optical carrier wave, which is another exampleof a temperature sensitive photonic device. Die 330 also includes anoptical waveguide 315 configured to carry an optical carrier wave thatis modulated by the carrier wave modulator 320. It should be understoodthat the described temperature-sensitive photonic devices, structures,and arrangements are merely exemplary, and the embodiments below may beapplied in connection with any known temperature-sensitive photonicstructures, including, e.g., temperature-sensitive photonic detectors,optical waveguides, filters, or other structures.

As shown in FIG. 3B, the photonic devices, such as input device 310,optical waveguide 315, and waveguide modulator 320, as well as heatingdevice 312, may be formed in an integrated circuit. Die 330 includes asubstrate 332, which may be, for example, a bulk region of thermallyconductive silicon. Although not shown in FIG. 3B, substrate 332 mayoptionally include an insulator or “BOX” region 233 (FIG. 2B) composedof an electrically insulating material, such as SiO₂, providing asilicon-on-insulator (SOI) substrate. Photonic devices including inputdevice 310, optical waveguide 315, and waveguide modulator 320 areformed in a device formation region 335 above a portion of the substrate332. Device formation region 335 may include a silicon area for deviceformation, and a cladding material, such as SiO₂, to serve as claddingfor an optical waveguide 315 that surrounds a silicon inner core and tooptically and/or electrically insulate one or more of the photonicdevices. Die 330 also includes one or more interlevel dielectric (ILD)regions 336 having, for example, SiO₂ as the lowest layer (i.e., thelayer closest to device formation region 335).

As shown in FIGS. 3A and 3B, a thermal isolation region 340 is formednear or adjacent to heating device 312. Thermal isolation region 340 canbe provided by forming an area of low thermal conductivity material indie 330. For example, as shown in FIG. 3B, thermal isolation region 340can include low thermal conductivity material formed in an area of thesubstrate 332 below heating device 312. Thermal isolation region 340prevents dissipation of heat from heating device 312 into the thermallyconductive substrate 332. Thermal isolation region 340 may extend beyondthe region of substrate 332 under heating device 312 into an area underone or more temperature sensitive photonic devices in die 330, such asunder carrier wave modulator 320, in order to thermally isolate thetemperature sensitive photonic devices from thermal changes in substrate332. In addition, one or more additional thermal isolation regions 341may be formed in an area of substrate 332 under temperature-sensitivephotonic devices in die 330, in order to thermally isolate thetemperature sensitive photonic devices from thermal changes in substrate332. For example, a second thermal isolation region 341 may be formed inan area of substrate 332 under input device 310.

Thermal isolation regions 340, 341 may be composed of, for example, alow thermal conductivity material, such as a material having lowdielectric constant relative to the silicon of the substrate 332. Forexample, the dielectric constant k of silicon used to form substrate 332of die 330 is typically approximately 11.7, and the SiO₂ used to formthe insulator region 333 (FIG. 4B) of an SOI substrate, the cladding indevice region 335, and/or the ILD region 336, is approximately 3.9.Techniques discussed below may provide a low thermal conductivitymaterial 340 having a dielectric constant k in a range of approximately1.0 to 3.8. Low thermal conductivity material 340 may also include athermal conductivity that is lower than silicon used in the die 330. Forexample, low thermal conductivity material 340 may have a thermalconductivity less than approximately 0.006 W/cm° C.

One technique for forming low thermal conductivity material for athermal isolation region 340, 341 is by doping an area of silicon orSiO₂ with a lower dielectric constant material than the material beingdoped. Examples of materials having lower dielectric constants thansilicon and SiO₂ include fluorine and carbon. Doping silicon or SiO₂with these or other lower dielectric constant materials reduces thedielectric constant of the doped material, thereby reducing the thermalconductivity of the doped material.

Another technique for forming low thermal conductivity material is tocreate an area of porous SiO₂. Certain techniques for depositing SiO₂,such as plasma enhanced chemical vapor deposition (PECVD), form SiO₂with voids or pores filled with ambient air or other gaseous material(referred to as porous SiO₂). The voids in porous SiO₂ may have adielectric constant of nearly 1.0, resulting in porous SiO₂ having alower dielectric constant than non-porous SiO₂. Further, porous SiO₂ maybe doped with a low dielectric constant dopant, such as fluorine orcarbon, to further reduce the thermal conductivity.

Another technique for forming low thermal conductivity material is todeposit SiO₂ using a spin-on deposition technique (referred to as“spin-on SiO₂”). Spin-on deposition techniques are typically used insilicon manufacturing processes to deposit materials such asphotoresist, while techniques such as chemical vapor deposition aretypically used to deposit SiO₂. Some examples of spin-on low dielectricconstant polymers include polyimide, polynorbornenes, benzocyclobuten,and/or polytetrafluoroethylene (PTFE). Spin-on deposition of SiO₂results in the SiO₂ having a lower dielectric constant, and lowerthermal conductivity. Further, SiO₂ deposited using a spin-on depositiontechnique may be doped with a low dielectric constant dopant, such asfluorine or carbon, to further reduce the thermal conductivity.

FIG. 3C shows another embodiment of photonic devices formed in a die 330including a thermal isolation region 342. Thermal isolation region 342includes a low thermal conductivity material formed in an area ofsubstrate 332 extending beneath heating device 312, input device 310,optical waveguide 315 and carrier wave modulator 320. Thermal isolationregion 342 provides thermal isolation of substrate 332 from heatingdevice 312, as well as thermal isolation of input device 310, opticalwaveguide 315 and resonant carrier wave modulator 320 from temperaturevariations in substrate 332. Other methods known in the art ofdepositing low thermal conductivity material may also be used.

More than one of the techniques described above for forming a lowthermal conductive material may be used in combination to providethermal isolation in die 330. For example, FIG. 3D illustrates across-sectional view of die 330, where a first thermal isolation region344 is formed under heating device 312 and extends under one or more ofthe photonic structures 310, 315, 320. A second thermal isolation region346 is formed within the first thermal isolation region 344, localizedunder heating device 312. Second thermal isolation region 346 may have alower thermal conductivity than the material of the first thermalisolation region 344. For example, first thermal isolation region 344may be composed of a porous SiO₂ material or an SiO₂ material depositedusing a spin-on deposition technique, and the porous or spun-on materialin the second thermal isolation region 346 is doped using, e.g., afluorine or carbon dopant to provide a lower thermal conductivity.

FIG. 3E shows another embodiment of photonic devices formed in a die 330including a thermal isolation region. Die 330 includes a resonantcarrier wave modulator 320 integrated with a heating device 312. In FIG.3E, device formation region 365 includes a low thermal conductivitymaterial. For example, after the formation of silicon photonic devicesabove substrate 332, outer cladding for optical waveguide 315 andinsulating material for the other photonic devices in die 300 can beprovided in device formation region 365 by depositing spin-on SiO₂ orporous SiO₂ in device formation region 365. Alternatively, portions orall of a region of SiO₂ material used to form device formation region365 may be doped with fluorine or carbon. Forming device formationregion 365 with a low thermal conductivity material reduces undesirabledissipation of heat from heating device 312, and thermally isolatestemperature-sensitive photonic devices in die 330 from temperaturevariations in substrate 332 and/or interlevel dielectric 336.

Further, die 330 includes an insulation region 363 providing asilicon-on-insulator substrate, where the insulation region 363 isformed with low thermal conductivity material. For example, insulationregion 363 may be formed from spin-on SiO₂ or porous SiO₂, or from SiO₂doped with fluorine or carbon. Forming insulation region 363 with a lowthermal conductivity material further reduces undesirable dissipation ofheat from heating device 312, and thermally isolatestemperature-sensitive photonic devices in die 330 from temperaturevariations in substrate 332.

FIGS. 4A-4E illustrate examples of manufacturing processes forembodiments of die 330 including thermal isolation regions. In FIG. 4A,a silicon substrate 332 is provided. In FIG. 4B, an optional insulation333 may be formed over silicon substrate 332, such as by thermallygrowing or depositing SiO₂ over substrate 332 using a chemical vapordeposition or other known process, to provide a silicon-on-insulatordie. In another embodiment, a portion or all of insulation 333 in FIG.4B is formed from a low thermal conductivity material. For example, aportion or all of insulation 333 can be formed using a plasma enhancedchemical vapor deposition (PECVD), to form porous SiO₂, or by depositingSiO₂ using a spin-on deposition technique. In other embodiments, theinsulation 333 may be omitted.

In FIG. 4C, a trench 351 may be formed, for example by etching, insubstrate 332, or, if present, into insulation 333 for an SOI substrate.Trench 351 can be formed using, for example, a dry etch process or otherknown etching techniques. In FIG. 4D, a layer 355 of low thermalconductivity material is deposited over substrate 332 and into trench351. In one embodiment, the low thermal conductivity material is SiO₂doped with a lower dielectric constant dopant, e.g., fluorine or carbon,that is deposited using standard techniques, such as chemical vapordeposition. In another embodiment, the low thermal conductivity materialis porous SiO₂ deposited using a PECVD technique. In another embodiment,the low thermal conductivity material is SiO₂ deposited using a spin-ontechnique. In FIG. 4E, the excess low thermal conductivity material fromlayer 355 is removed from substrate 332 using known techniques, such asa chemical or mechanical polish, leaving thermal isolation region 340.

In FIG. 4F, photonic devices, such as an input device 310, an opticalwaveguide 315, a carrier wave modulator 320, or other photonic devicesare formed above substrate 332. Heating device 312 is formed abovethermal isolation region 340.

In FIG. 4G, device formation layer 335 is completed by depositinginsulating and cladding material, such as SiO₂, surrounding the formedphotonic devices 310, 315, 320. In another embodiment, a low thermalconductivity material, such as a doped SiO₂, porous SiO₂, or spin-onSiO₂ is used as an insulating and cladding material to complete thedevice formation region. Other regions, such as an ILD region 336 (FIG.3B), can then be formed over the device formation region 335.

FIG. 5 shows another embodiment of a die including temperature isolationregions. In FIG. 5, a shallow trench isolation (STI) region 350 isformed in an area of substrate 332 under heating device 312. In existingCMOS manufacturing processes, STI regions composed of SiO₂ are used toelectrically isolate devices within die 330. In the embodiment shown inFIG. 5, an STI region 350 composed of a low thermal conductivitymaterial, such as a doped SiO₂, porous SiO₂, spin-on SiO₂, or other lowthermal conductivity material, is formed in an area of substrate 332below heating device 312. STI region 350 thermally isolates substrate332 from heating device 312, and reduces unwanted dissipation of heatfrom heating device 312 into substrate 332. In addition to, or insteadof, STI region 350 under heating device 312, die 330 can include an STIregion 352 formed in an area of substrate 332 below one or more photonicdevices, such as in an area below input device 310, in order tothermally isolate the photonic devices from thermal variations insubstrate 332. STI regions 350, 352 can be partially, or substantiallycompletely, formed from low thermal conductivity material.

FIGS. 6A-6E illustrate additional embodiments of die 430 includingtemperature sensitive photonic devices and thermal isolation regions.Exemplary photonic structures shown in FIGS. 6A-6E include an inputdevice 410, an optical waveguide 415, and/or a resonant carrier wavemodulator 420, although it should be understood that any other photonicstructures may be included, such as a photonic detector or opticalfilter. Die 430 also includes one or more heating devices 412, which maybe separate heating elements or integrated with a carrier wave modulator420. Die 430 includes a substrate 432, device formation region 435, andILD region 436, which may be formed of similar materials discussed abovein connection with FIGS. 3A-3E. Although not shown for purposes ofclarity, substrate 432 may optionally include an insulation region 233(FIG. 2B) providing an SOI substrate.

In FIGS. 6A and 6B, a thermal isolation region is provided by forming aphysical gap 440 to create a thermal break between heating device 412and adjacent material. At the temperature and dimensions used in siliconphotonic systems, conduction is the most efficient heat flux mechanism.A thermal break involves removing a portion of the conductive material,leaving a gap or void that prevents thermal conduction across the gap.In another embodiment, gap 440 is formed in a portion of an insulationregion 233 (FIG. 2A) of an SOI substrate underlying heating device 412.Gap 440 may be localized in an area of substrate 432 under heatingdevice 412, or may extend into the area of substrate 432 under one ormore photonic devices, such as carrier wave modulator 420, in order tothermally isolate the photonic devices from thermal variations insubstrate 432.

Because a thermal break drastically reduces the dissipation of heat q inits respective direction, a relatively small gap is sufficient forthermal isolation of substrate 432. Gap 440 may be, for example, an airgap in substrate 432 on the order of a few nanometers, so long as thegap is sufficient to prevent thermal conduction.

In another embodiment shown in FIG. 6C, a gap 452 is provided within ashallow trench isolation (STI) region 450. STI region 450 may becomposed of an electrically insulating material, such as SiO₂ or otheroxides. STI region 450 with gap 452 reduces the dissipation of heat qfrom heating device 412 into substrate 432 through STI region 450.Alternatively, STI region 450 may be composed of a low thermalconductivity material, as discussed above in connection with FIG. 5,such that gap 452 and STI region 450, in combination, provide forthermal isolation of substrate 432 from heating device 412.

In another embodiment shown in FIG. 6D, die 430 includes a gap 540surrounding heating device 412 on multiple sides, including in portionsof substrate 432, device formation region 435, and ILD region 436surrounding heating device 412. Gap 540 provides thermal isolation toreduce heat dissipated by heating device 412 into substrate 432, as wellas into ILD region 436 and other adjacent portions of device formationregion 435 that are not meant to receive active temperature control fromheating device 412, allowing for more efficient active temperaturecontrol and reduced global temperature flux. Gap 540 also improves thethermal isolation of overlying circuitry (i.e., formed in or above ILDregion 436) and neighboring circuitry (i.e., in adjacent portions of die430) from heating device 412, reducing the thermal effect of heatingdevice 412 on the overlying and neighboring circuitry. Because thedissipation of heat q in the direction of the temperature-sensitivephotonic structures that heating device 412 is intended to activelyregulate is still desirable, gap 540 should provide little or noseparation between heating device 412 and device formation region 435 inthe direction of thermally-sensitive photonic structures to which it isproviding active temperature control, such as carrier wave modulator420.

As also shown in FIG. 6D, a second thermal isolation region can beprovided by forming a separate gap 542 in the area of substrate 432under one or more of the temperature-sensitive photonic devices, such asunder carrier wave modulator 420. The second thermal isolation region542 thermally isolates carrier wave modulator 420 from thermalvariations in the substrate 432.

In the embodiments described above in connection with FIGS. 6A-6D, therespective gap may be etched such that sufficient material remains toprovide mechanical support for overlying devices, such as heating device412. As discussed above, a gap on the order of a few nanometers may besufficient to provide thermal isolation of surrounding materials.Furthermore, as shown in FIG. 6E, interconnects 551, 552 that are usedto provide electrical energy to heating device 412 extend into thematerial of device formation region 432 and/or ILD region 436 thatsurrounds gap 540. Interconnects 551, 552 provide mechanical support forheating device 412 despite a gap 540 surrounding substantially all sidesof heating device 412. Similarly, signal lines 421, 422 that areconfigured to provide an electrical data signal to carrier wavemodulator 420 extend into the material of device foi region 432 and/orILD region 436, providing mechanical support for carrier wave modulator420 despite the underlying gap 542 in substrate 432. Because heatingdevice 412, carrier wave modulator 420 and other photonic devices aremanufactured on a microscopic scale, the devices have relatively littlemass, and thus the interconnects 551, 552 for heating device 412 andsignal lines 421, 422 for carrier wave modulator 420 can providemechanical support for the respective devices. Other photonic deviceswill typically include similar physical connections that extend into thesurrounding material of device formation region 432 and/or ILD region436, providing mechanical support for the photonic devices.

FIGS. 7A-7E illustrate a process for forming a thermal isolation regionincluding a gap 440. In embodiments shown in FIGS. 7A-7E, a gap 440 isformed in substrate 432, by performing an etching process in substrate432 after completing formation of the device formation region 435,including photonic devices such as input device 410, optical waveguide415, and carrier wave modulator 420, and heating device 412. It shouldbe understood, however, that gap 440 may instead be formed in substrate432, or in an insulator region 233 (FIG. 2A), prior to the formation ofphotonic devices 410, 415, 420 and heating device 412 in deviceformation region 435, such as by performing a vacuum or etching processafter providing the substrate 432.

In FIG. 7A, photonic devices, such as an input device 410, an opticalwaveguide 415, a carrier wave modulator 420, or other photonic devices,and a heating device 412 for providing active temperature control, areformed in device formation region 435 of a die 430. A material such asSiO₂ is deposited to complete the device formation region 435.

In FIG. 7B, an opening 448 is formed starting at a top surface of deviceformation region 435 in a location proximate to, but not directly over,heating device 412. Opening 448 traverses device formation region 435,and is formed using, for example, reactive ion etching or othertechniques.

In FIG. 7C, gap 440 is formed through opening 448, such as by using apulse etch process where the etching fluid passes through opening 448,or through a dry etch process.

After completion of the etching process, opening 448 may be filled witha dielectric material, as shown in FIG. 7D. For example, opening 448 canbe filled with SiO₂ through a chemical vapor deposition process.Alternatively, opening 448 may be left open during further processing.

In FIG. 7E, an ILD region 436 is formed over the device formation region435 and opening 448. It should be understood that the process describedin connection with FIGS. 7A-7E could also be performed after formationof an ILD region 436 over device formation region 435.

It should also be understood that, while FIGS. 7A-7E show the formationof a gap 440 in a region of substrate 432 under heating device 412, thetechniques described in connection with FIGS. 7A-7E can be used to forma gap 440 in any desired location in die 430, such as under one or moreof the photonic devices, in device formation region 435 or ILD region436, in an STI region, or in the various locations described inconnection with FIGS. 6A-6E.

Die including the thermal isolation regions described above may beimplemented as part of an integrated circuit. The correspondingintegrated circuit may be utilized in a typical processor system. Forexample, FIG. 8 illustrates a typical processor system 700 including aprocessor and/or a memory device employing one or more of the thermalisolation structures and techniques described in connection with FIGS.3-7.

Processor system 700, which may be, for example, a computer system,includes a central processing unit (CPU) 760, such as a microprocessor,a digital signal processor, or other programmable digital logic devices,which communicates with one or more input/output devices 780 over a bus750. A memory device 770 also communicates with CPU 760 over bus 750,such as through a memory controller. The memory device 770 may include,for example, RAM, a hard drive, a FLASH drive or removable memory. Inthe case of a computer system, processor system 700 may include otherperipheral devices, such as removable media devices, that communicatewith CPU 760 over bus 750. In some embodiments, memory device 770 may becombined with CPU 760 as a single integrated circuit. Integratedcircuits implementing the thermal isolation structures and techniquesdescribed herein may be used to provide efficient and reliable thermalisolation of photonic devices, electronic devices, and materials in oneor more of CPU 760, memory device 770, input/output devices 780, or inconnection with many other aspects of a processor system 700.

The above description and drawings are only to be consideredillustrative of specific embodiments, which achieve the features andadvantages described herein. Modification and substitutions to specificprocesses, components, and structures can be made. For example, itshould be understood that appropriate types of semiconductor materialsand memory elements other than those specifically described inconnection with the above embodiments may be used. Accordingly, theembodiments of the invention are not to be considered as being limitedby the foregoing description and drawings, but only by the scope of theappended claims.

Described embodiments include photonic integrated circuits and systemswith photonic devices, including thermal isolation regions for thephotonic devices. Methods of fabricating such circuits and systems arealso described.

We claim:
 1. A method of forming an integrated circuit, the methodcomprising: providing a substrate; forming a trench in an upper surfaceof the substrate; forming a temperature-sensitive photonic device abovethe substrate and separated from the trench by a portion of thesubstrate; forming a heating device above the substrate and over thetrench such that the trench extends under the entirety of the heatingdevice, wherein the heating device is configured to dissipate heattoward the photonic device; and forming a thermal isolation region inthe trench, wherein the thermal isolation region reduces dissipation ofheat from the heating device into the substrate in the direction of theisolation region, wherein the trench does not extend underneath anyportion of the temperature-sensitive photonic device.
 2. The method ofclaim 1, wherein forming the thermal isolation region comprises forminga region of low thermal conductivity material that is less thermallyconductive than material in the area of the die in which the thermalisolation region is formed.
 3. The method of claim 2, wherein formingthe region of low thermal conductivity material comprises: etching thetrench in the substrate; and depositing the low thermal conductivitymaterial in the trench.
 4. The method of claim 3, wherein depositing thelow thermal conductivity material in the trench comprises doping silicondioxide with a material having a lower dielectric constant than thesilicon dioxide.
 5. The method of claim 3, wherein depositing the lowthermal conductivity material in the trench comprises depositing poroussilicon dioxide.
 6. The method of claim 3, wherein depositing the lowthermal conductivity material in the trench comprises depositing silicondioxide using spin-on deposition.
 7. The method of claim 2, whereinforming a region of low thermal conductivity material comprises formingan insulating layer between the substrate and the heating device.
 8. Themethod of claim 7, wherein the insulating layer comprises an oxide dopedwith a material having a lower thermal conductivity than the oxide. 9.The method of claim 7, wherein the insulating layer comprises poroussilicon dioxide.
 10. The method of claim 7, wherein the insulating layercomprises spin-on silicon dioxide.
 11. The method of claim 1, whereinforming the thermal isolation region comprises forming a physical gapbetween the heating device and material adjacent to a portion of theheating device.
 12. The method of claim 11, wherein the gap is formed inthe substrate.
 13. The method of claim 11, wherein the gap is formed ina region above the substrate.
 14. The method of claim 11, whereinforming the gap comprises performing a silicon etch to remove a portionof material adjacent to the heating device.
 15. The method of claim 11,wherein forming the gap comprises performing a vacuum process to removea portion of material adjacent to the heating device.
 16. The method ofclaim 1, wherein the thermal isolation region is formed prior to formingthe heating device.
 17. The method of claim 1, wherein the thermalisolation region is formed after forming the heating device.
 18. Themethod of claim 17, further comprising: forming an opening near theheating device, wherein the opening passes into the substrate; andforming the thermal isolation region under the heating device.